Project Summary Many human cancers, including breast and pancreatic cancers, harbor aberrant Stat3 that functions as a master regulator of events that promote tumor formation and progression. Blocking aberrant Stat3 activity induces cancer cell growth inhibition and apoptosis, and tumor regression in mouse models. Given the number of human tumors with aberrant Stat3 activity, there is a high commercial potential for Stat3 inhibitors as novel anticancer drugs. Moreover, effective and selective Stat3 inhibitors will provide excellent molecular probes to interrogate Stat3 signaling to advance our understanding of the molecular mechanisms by which this protein mediates carcinogenesis, and provide new directions to thwart the functions of the aberrant protein. By computational modeling that exploited the key structural requirements for Stat3 activation, together with in silico screening of the NCI's Diversity and Plated sets chemical libraries, we identified agents, NSC 42067, NSC 59263 and NSC 74859, as binders of Stat3-SH2 domain and inhibitors of Stat3 DNA-binding activity (IC50 of 65-86 <M). In preliminary work, agents induced selective antitumor cell effects and breast tumor regression in xenografts, thereby providing the rationale for exploring them as molecular probes and for novel anticancer drug design. While agents are predicted to interact with the Stat3-SH2 domain, the exact mode of Stat3 inhibition is unknown. Our overall objective is to apply an integrated multi-disciplinary approach to critically explore the structural, molecular and biological properties of these molecules and to capitalize on their structural properties to facilitate the design of potent, selective and efficacious analogs. We hypothesize 42067, 59263, 74859, or analogs interact with the Stat3-SH2 domain, inhibit Stat3 function, and induce the regression of tumors harboring aberrant Stat3. The hypothesis will be tested by the following Specific Aims: (1) To probe the biochemical interactions of 42067, 59263, 74859, or analogs with Stat3 in vitro. Perform biochemical, biophysical, and structural analyses of interactions with Stat3;(2) To use structural information to optimize molecules for enhanced potency and selectivity. Use structural information from Aim 1 and modeling to facilitate the design of more potent, selective, and efficacious analogs;(3) To define the biochemical, molecular and biological effects of select agents in breast and pancreatic cancer cells in vitro;and (4) To test select agents for effects on breast and pancreatic tumor growth, metastasis, and angiogenesis in xenografts. Studies will measure antitumor effects and investigate underlying molecular mechanisms in vivo. These are important studies that will define the molecular and biological properties of agents, and identify potent Stat3-selective inhibitors with antitumor activity for development into novel anticancer drugs. PUBLIC HEALTH RELEVANCE: Many human cancers harbor abnormal Stat3 activity that functions as a master regulator of events that promote tumor formation and progression. Blocking abnormal Stat3 activity induces cancer cell growth inhibition, apoptosis, and tumor regression in mouse models. Effective and selective Stat3 inhibitors are excellent research tools to interrogate Stat3 signaling to advance our understanding of the mechanisms leading to cancer, and represent potential new anticancer agents. Our proposal defines the molecular and biological properties of novel agents as Stat3-selective inhibitors that possess antitumor activity and that may be suitable for development into novel anticancer drugs.